The presence of inclusions in superalloy investment castings is a concern of gas turbine engine manufacturers as the engine operating conditions, especially temperature, have become more severe. As is known, these inclusions can adversely affect the mechanical properties of the casting and, if present at a critical size, can cause catastrophic failure of the castings under the high temperature and stress conditions of engine service. Moreover, the adverse effect of inclusions on the casting properties may be exacerbated as the section size (e.g., wall thickness) of the castings is reduced for weight savings purposes. The presence of unacceptable levels and/or sizes of inclusions in investment castings results in excessive scrap rates that increase the overall cost of producing castings for use in gas turbine engines. The inclusion problem worsened as remelt vacuum levels and leak rates of casting equipment were improved.
Non-metallic and dross type inclusions can originate from several sources during the investment casting operation. For example, inclusions can be introduced by the master alloys used, by remelting in a ceramic crucible, by the remelting environment, and by mold/melt reactions that can occur during solidification, especially during the relatively long mold/melt contact periods required in the production of directionally solidified and single crystal castings.
In efforts to produce cleaner castings (i.e., castings with lower non-metallic inclusion levels), superalloys are typically remelted and cast under relatively high vacuum conditions (e.g., &lt;10.sup.-3 torr) to reduce the presence of residual oxygen and other gases in the remelting/casting atmosphere and/or in the melt. Typical remelting procedures employed include vacuum induction melting in a refractory crucible and "cold hearth" melting including vacuum arc, induction skull and electron beam melting in a water cooled metallic (e.g., copper) crucible. Despite this effort, unacceptably high inclusion levels may still be experienced in the production of certain superalloy investment castings that include one or more relatively volatile alloying elements, such as Cr and Al, especially during the melting of successive alloy charges in the same crucible.